Explanation:
When does the line between day and night become vertical?
Tomorrow.
Tomorrow is an
equinox on planet Earth,
a time of year when day and night are most nearly equal.
At an equinox, the
Earth's terminator --
the dividing line between day and night -- becomes vertical and connects the
north and
south poles.
The above time-lapse video
demonstrates this by displaying an entire year on planet Earth in twelve seconds.
From geosynchronous orbit, the
Meteosat satellite recorded
these infrared images
of the Earth every day at the same
local time.
The video started at the
September 2010
equinox with the terminator line being vertical.
As the
Earth revolved around the Sun, the
terminator was seen to tilt in a way that provides less daily sunlight
to the northern hemisphere,
causing winter
in the north.
As the year progressed, the
March 2011
equinox arrived halfway through the video, followed by the terminator tilting the other way,
causing winter
in the southern hemisphere -- and summer in the north.
The captured year ends again with the
September equinox,
concluding another of billions of trips the Earth has taken
-- and will take -- around the Sun.